There is an increasing demand for cryogenic air separation plants to produce very large quantities of oxygen for use for example in direct reduction steel making processes, coal-gasification processes, and partial oxidation processes in which natural gas is converted to synthesis gas.
Most modern commercial air separation plants employ a high pressure rectification column having its upper end in heat exchange relationship with the lower end of the lower pressure rectification column. Cold compressed air is separated into oxygen-enriched and nitrogen-enriched liquids in the higher pressure column, and these liquids are transferred to the lower pressure column for separation into nitrogen-enriched and oxygen-enriched products. Large quantities of energy are required to compress the feed air. U.S. Pat. No. 3 731 495 discloses a process for reducing the external power consumption of the process. The process employs a nitrogen-quenched power turbine. A portion of the compressed feed air is mixed with fuel and combusted. A hot combustion mixture is then quenched with waste nitrogen-rich gas from the lower pressure rectification column and the resulting gaseous mixture is expanded in a power turbine. The expansion provides energy to compress the feed air. A major disadvantage of this process is that the pressure of the gaseous mixture expanded in the power turbine can be no higher than that of the waste nitrogen mixed with the combustion gases. As pointed out in U.S. Pat. No. 4 224 045, commercially available power turbines have optimum inlet pressures in excess of the optimum operating pressure of the lower pressure rectification column. Accordingly, U.S. Pat. No. 4 224 045 proposes compressing waste nitrogen from the lower pressure rectification column prior to using it to quench the combustion mixture.
Additional work is thus required to compress the nitrogen from a pressure just above one atmosphere to a pressure in excess of ten atmospheres.